PRESS BULLETIN 142
florlda Agricullural Experiment Stalion
By B. F. Floyd
Melanose is a disease of the fruit, leaves, and young branches of citrus
trees. No organism has been proved to be the cause of this disease. Under
the microscope, it nearly always shows the development .of a corky layer
of cells just beneath the epidermis, which indicates some outside influence
as causing the trouble. The disease is associated with a weakened condition
of the tree, and has been found to reach its greatest development on
neglected trees, or on those otherwise diseased.
In 1894, Swingle and Webber found that melanose could be controlled
by the use of Bordeaux mixture or ammoniacal solution of copper carbonate.
They advised two sprayings, "the first about two weeks after the flowers
have fallen, and the second about a month later. A Vermoyel nozzle, and
a pump giving a good spray, should be used. The spray must be applied
to the fruits in the form of a fine mist covering then thinly and evenly."
Since these sprays are fungicides, they will kill off the natural fungus
enemies of the purple scale and allow this scale to develop. Hence these
sprayings should be followed at intervals with some good insecticide until
the friendly fungi have returned.
Melanose is found on nearly all varieties of citrus. Its effects extend
to only a small distance beneath the skin. It harms the fruit by the mark-
ings, which cause such oranges or grapefruit to be graded as second-class.
The disease shows wn to black, more or less rounded or conical
elevations from the epidermis. These elevations vary in size, from mere
points to one-sixteenth of an inch in diameter. In gross appearance they
are much like specks of burned sugar. They sometimes run together form-
ing irregular raised areas. Frequently there is an arrangement of the ele-
vations in curves that may meet forming rings.
The markings on the fruit are not so much raised as those on the stems
and leaves. The margins of many will be seen under a hand lens to have
February 12, 1910
broken away from the surrounding healthy tissue, and the wax-like mass
to have shrunk towards the center, forming a scale-like body. This ap-
pearance has caused the disease to be sometimes mistaken for San Jose
scale. In other cases, the cracks occur in lines like the cracking of dry
mud. In many cases no cracking at all occurs.
The markings on the leaves and young branches are more raised, and
usually do not have the scaly appearance often seen on the fruit. They
frequently run together, the stems being more or less completely covered.
The elevations appear on one or both surfaces of the leaves, but reach their
greatest development on the midribs. In severe cases, the leaves may
be deformed and partially lose their green color. Large leafy quick-growing
branches on the lower half of the tree are likely to be most commonly and
Some of the symptoms of dieback (such as markings on stained terminal
branches, on ammoniated fruits, or in connection with bark excretions)
sometimes resemble the markings caused by melanose. However, the mark-
ings on the stained terminal branches are usually not raised and are not
made up of collections of small spots. The same is true of the markings on
ammoniated fruit; while gum masses also occur in the angles of the seg-
ments of ammoniated fruits, but not in melanosed fruits. The bark excre-
tions of die-back are usually masses of a gum-like substance which does
not appear in melanose.
Melanose starts its development only on young tissue, and first appears
as somewhat sunken points that can barely be seen with the unaided eye.
Under a hand lens each of these sunken points is seen to consist of a
brownish wax-like mass. As the markings develop, they become raised
areas. A yellowing of the tissue surrounding these spots sometimes appears.
Melanose occurs in all parts of the State, even in the most isolated lo-
calities. It has been found prevalent upon sour orange trees at Gulf Ham-
mock, Fla., where there were no other groves within a radius of fifteen
miles. During the present season melanose has increased in prevalence
and severity. Its presence on the fruit has caused a money loss that will
be large in the total for the year. Like many other diseases its prevalence
varies from year to year, probably from climatic conditions. During the
past two years it has been increasing in severity. Melanose is also pre-
valent in the West Indian Islands; and in Australia the citrus trees have
been reported as suffering so much from this disease that the crop was
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